Radio receiver



Malmhv 29, 1938.

| E. BARTON RADlo RECEIVER Filed Deo. l, 1933 2 Sheets-Sheet l March 29,1938. E. BARTON RADIO RECEIVER Filed Deo. 1, 1955 2 Sheets-Sheet` 2 L. Ww 1 1 w n Mau 4 MM flow/2 9 1 9. 1 1 1 -ill z ww m Wad u Niv. fr 1ATTORNEY Patented Mar. 29, 1938 YUNITED STATES PATENT CFFICE RADIORECEIVER Delaware Application December 1, 1933, Serial No. 700,471

16 Claims.

'Ihe present invention relates to radio receivers and particularly totuning control devices and circuits therefor.

An objectionable feature of present high quality radio receivers is thatthey are so selective that they are somewhat difficult to tune.Furthermore, it is a recognized fact that it is desirable to make thetuning of radio receivers still more exact, preferably by means of avery sharp noise suppressor circuit, the noise suppressor circuitserving both for eliminating background noise when tuning betweenstations and for insuring the proper tuning of the receiver. Theselectivity provided by a noise suppressor may be referred to as theapparent selectivity since a noise suppressor increases only thesharpness of tuning and does not increase the selectivity of the signalchannel.

The advantage which is obtained by increasing the apparent selectivityof the receiver is usually oifset, however, by the fact that the tuningbecomes very critical, and it becomes difficult for the average personto stop the tuning control at the proper tuning point, the proper tuningpoint being both the correct position for tuning in a signal withoutdistortion and the correct position for releasing the noise suppressorso that the signal is heard.

It is, accordingly, an object of my invention to provide a tuning devicefor radio receivers which will make them less difficult to tune thanwhen they are equipped with a conventional tuning control device.

It is a further object of my invention to provide improved tuning meansfor preventing a radio receiver from being tuned inaccurately during thereception of a signal.

More specifically it is an object of my invention l to provide automatictuning control means which permits a sharply tuned radio receiver to betuned easily to the exact station position.

It is a still further object of my invention to provide a radio receiverwhich may be tuned exactly to an incoming signal without the aid ofeither sight or hearing, and without preselecting the stations.

A still further object of my invention is to provide a radio receiverhaving an improved indieating device which will indicate when thereceiver is tuned exactly to the carrier frequency of an incoming radiosignal whereby distortion of the signal due to inaccurate tuning isavoided.

In practicing my invention, in one embodiment thereof I provide asuperheterodyne receiver with a very sharp noise suppressor circuit andwith an (Cl. Z50-20) automatic volume control circuit so connected thatit prevents the signal input to the noise suppressor from exceeding apredetermined value. 'Ihe tuning control device of the receiver isprovided with a magnetic brake which is controlled ,5 by the noisesuppressor circuit. When the` receiver is tuned exactly to an incomingsignal, the noise suppressor circuit functions to permit the signal topass through the receiver and to energize the magnetic brake, wherebythe tuning control device is instantly locked in the correct tuningposition.

Before the receiver is tuned to another station position, the magneticbrake must be released. This may be accomplished by means of a releasecircuit which may be closed by some predetermined movement of the tuningknob, such as an axial movement toward the panel of the receiver, or thebrake may be released by means of a circuit which automaticallyde-energizes the brake 20 after it has been energized a predeterminedlength of time.

Other objects, features and advantages of'my invention will appear fromthe following description taken in connection with the accompanying :2-5drawings in which Figure 1 is a circuit and schematic diagram of oneembodiment of my invention;

Fig. 2 is a front view of the tuning dial and magnetic brake shown inFig. 1; f3@

Fig. 3 is a side view of the structure shown in Fig. 2; and

Fig. 4 is a view illustrating a modified form of my invention.

Referring to Fig. 1, an embodiment of my in- L35 vention is shownapplied to a superheterodyne receiver which comprises a radio frequencyamplii'ler I having an input circuit coupled to an antenna 3 through aradio frequency transformer 5. The output circuit of the amplifier I iscoupled E4.0 to a rst detector 'I through a radio frequency transformer9.

The secondary windings of the transformers 5 and 9 have variable tuningcondensers II and I3, respectively, connected thereacross in the usual,45 manner, for tuning the input circuitsy of the amplier I and firstdetector 1 to the desired incoming signal.

An oscillator I5 is coupled to the first detector 1 for the purpose ofheterodyning an incoming 50 signal to a lower intermediate frequencysignal. The frequency of the oscillator output may be varied by means ofa variable tuning condenser l1.

The tuning condensers II, I3, and I1 are pref.- ,55

erably units of a gang condenser, and may be varied simultaneouslythrough a common tuning control indicated by a dotted line |9.

'I'he intermediate frequency output of the first detector is impressedupon the input circuit of an intermediate frequency amplifier 2| throughan intermediate frequency transformer 23, the primary and secondary 0fWhich are tuned to make the transformer function as a band-pass filterhaving a pass range of sufficient width to pass the intermediatefrequency carrier and. at least one signal side band.

It will be understood, of course, that the tuned' radio frequencycircuits of the amplifier I and detector 1 are also tuned broadly enoughto pass the carrier of the incoming radio signal and' at least one ofits side bands. Y

The output of the intermediate frequency amplifier 2| is fed to thesecond detector 25 throughk an intermediate frequency transformer 21tuned in the'same mannerfas the preceding transformer 23.

In the' particular embodiment. illustrated', the second detector 25 is adiode rectifier which is one unit of a double diode-triode vacuum tube29'.

One terminal ofthe secondary 3| of transformer 21 is connected' tothedetector diode plate 33, While the other terminal ofy the' secondary 3|isl connected to the cathode through a. resistor 3T, the resistor beingshunted by means of an intermediate' frequency bypass condenser 39.

The triode section 4| of the Vacuum tube 29 functions as the first audiofrequency amplifier. Its control grid 43 is connected to the plate endof the resistor 31, whereby any audio signal appearingacross theresistor 31 is impressed across the input of the triode section and anamplified audio signal appears in the plate circuit thereof. A resistor38" reduces' the intermediate frequency voltage' that may otherwise' getto the grid 43'.

The amplified audio sigr'ial: is' impressed upon the input circuit ofanother audio frequency amplifier stage comprising an electric dischargedevic'e such as a vacuum tube 45,- The signal? is impressed upon said'input circuit through a cou- `pling system which, in the embodimentshown, includes an audio frequency choke coil 41 inthe plate circuit ofthe trlode section 4|. The upper end of the chokecoil 41 is coupled to'a volume control resistor 49 through akcoupling condenser 5|, the lowerend of the resistor being connected to ground. The potentiometer 49maybe tapped to obtain the usual compensation of audio frequency signalsat various volume levels.

The control grid 53 of the amplifier 45 is connected to the volumecontrol resistor 49 through a variable volume control tap 55.

The output of the amplifier may be further ainplifed by any suitableaudio frequency amplifier, generally indicated at 51,- and supplied to aloudspeaker 59.

Y It will be noted thatl the input circuit of the audio frequencyamplifier 45 includes an electric discharge device 6| which may be avacuum tube of the pentode type. The cathode-anode circuit of the tube6I is connected between the cathode 63 of tube 45 and ground to functionas a selfbiasing resistor which may be given a very high resistance toblock the amplifier tube 45 when tuning between stations. In otherWords, the tube 6| applies noise suppressor control to the audiofrequency amplifier tube 45 by functioning as a self-biasing resistor.This feature of my invention is described and claimed my Patent l apentode section 12 consisting of the cathode 1|,

a control grid 13, a screen grid 15, a suppressor gri'd11?, and an anode19.

The input circuit of. the pentode section 12 is coupled't'o the resistor31 in any suitable manner for impressingthe low intermediate frequencyvoltage drop of the resistor across the pentode amplifier input circuit.In the drawings, this coupling device is shown as a small couplingcondenser 8|, having a comparatively low impedance tosignals at theintermediate frequency, but having an impedance` to audio frequencysignals sufficient to prevent appreciable audiosignal being applied tothe control grid 13.

A- coupling resistor 8-3 is provided which is connectedat one end to thecontrol gridv 13v through a conductor 851, and connectedv at the otherend tothe cathode 1|- through an audio-frequency bypass condenser 81.The coupling resistor 83 is shuntedby a variable condenser 89 which maybe adjusted for the purpose of controlling the input voltage to thepentode amplifier 12.

The plate circuit `of the pentode section 12 includes the primarywinding 6.6 of the transformer 61- anda resistor 9|, the primary windingand the resistor being connected in series, and the lower end of theresistor being connected to a positive point on the voltage divider 93of the power supply 95.

The secondary winding 68 of the transformer 61 is loosely coupled to theprimary winding 66, and both the primary and the secondary are tuned sothat the transformer is tuned very sharply to the intermediatefrequency.

The amplified intermediate frequency output of the pentode section 12 istransferred through the sharply tuned transformer 61 and impressed uponlthe elements 69 and 1| of the diode rectifier through a conductor 91which connects the diode plate 69 to one terminal of the secondarywinding 68, and through a connection from the cathode 1| to the otherterminal of the secondary 68, which may be traced from the cathode 1|through a self biasing resistor 99 to ground, through ground to thelower terminal of a resistor IBI, (which may have a` resistance of theorder of one megohm), and through the resistor |0| to the said otherterminal of the secondary 68.

The sole function of the diode rectifier 10 is to increase the negativebias on the control grid 13 of the pentode Sectio-n upon reception of aradio signal. Since a flow of current through the diode rectifier 10causes the upper end of the resistor ||l| to become negative, thecontrol grid 13 also becomes more negative because of its connection tothe negative end of the resistor |0| through a high impedance resistor|03.

Obviously an audio frequency signal will appear in the circuit ofrectifier 10 which should not be impressed upon the control grid 13 ofthe pentode section. This is prevented by the audio frequency bypasscondenser 81.

It will be noted that the self biasing resistor 99, the resistor |0|,and the plate resistor 9| are shunted by the usual intermediatefrequency bypass condensers |01, |05, and |09, respectively.

It will be apparent from the foregoing description that as soon as anintermediate frequency signal is impressed upon the input circuit o-fthe pentode amplifier 12, the control grid 13 of that amplifier willbecome more negative because of the current flow through thel circuit ofthe diode 10.

It may be noted that the change in bias on the control grid 13 depends,to a certain extent, upon the voltage drop produced in the self-biasingresistor 99, since this Voltage drop is in series with the voltage dropalong the resistor of the diode circuit. That these voltage drops are inseries will be made evident by tracing the direct current path of thepentode section input circuit. It may be traced from the control grid 13through the resistor 83, the high impedance resistor |03, through thediode circuit resistor 0| to ground, through ground to the self-biasingresistor 99, and through the self-biasing resistor 99 to cathode 1|.

When there is no incoming signal, the plate current of the pentodeamplifier portion 12 is large, since the only negative bias on thecontrol grid 13 is that due to the self-biasing resistor 99, and thisnegative bias is comparatively small. Consequently there is a largevoltage drop in the plate resistor 9|, whereby the upper end of theresistor 9| is at a fairly high negative potential with respect to theupper end of the Voltage divider 93. Also, the gain of amplifier 12 islow because of the low voltage on its anode 19.

As soon as the receiver is tuned to an incoming carrier wave, amplifier12 supplies signal energy to the diode 10, the voltage drop in the diodecircuit resistor |0| increases the negative bias on the control grid 13,whereby the plate current of amplifier 12 decreases and the upper end ofthe plate resistor 9| becomes less negative and the voltage applied toanode 19 is increased. It is the potential at this end of the plateresistor 9| which is utilized for actuating both the noise suppressorand the automatic tuning control, this point in the circuit beingidentified as point A on the drawings.

It should be noted that although the decrease in plate current tends tocause a reduction in the biasing voltage in the self-biasing resistor99, the increased negative bias provided by the current flow through thediode circuit is much greater than any decrease in bias due to reductionin plate current.

The above described action of the diode-pentode circuit whereby theupper end of plate resistor 9| becomes less negative in response toreception of a radio signal is preferably made a trigger -action bygiving plate resistor 9| a proper resistance value. This trigger actiondepends upon the fact that the increase in gain caused by the increasein the plate voltage is greater than the decrease in gain caused by theincrease in negative bias on the grid until a more or less definite biasis reached. In practice, a tube of the type known as RCA 2B7 has beenfound satisfactory for use as the tube 65.

A trigger amplifier employing a resistor in the plate circuit of apentode tube is described and claimed in my co-pending -applicationSerial No. 704,510, filed December 29, 1933, and assigned to the sameassignee as this application.

In order to provide the proper control voltages for the noise suppressorbiasing tube 6| and for the magnetic brake control circuit, which isdescribed hereinafter, a potentiometer is connected between the upperend of the plate resistor 9| and the negative end of the voltage divider93. This potentiometer consists of three resistor sections, R5, R6, andR1, the sections R5 and R1 having a high resistance value which may beof the order of one megohm, while the resistor section R6 preferably hasa lower value of the order of 100,000 ohms.

Referring now more specically to the biasing tube 6|, it has a cathode|l3, a control grid I5, a screen grid ||1, a suppressor grid ||9, and ananode 2| The anode |2| is connected to the cathode 63 of the audiofrequency amplifier 45, while the cathode ||3 is connected to ground,and through ground to the lower end of the volume control resistor 49.An audio frequency bypass condenser |23 (preferably having a capacity offrom 4 to 8 microfarads) is connected between the cathode ||3 and anode|2| for bypassing the audio frequency signal around the plate impedanceof the tube 6|. Thus it will be seen that the plate impedance of thetube 9| is connected in the cathode circuit of the audio frequency`amplifier 45 to act as a self-biasing resistor, while the audiofrequency signal appearing across the volume control resistor 49 isimpressed upon the input circuit of the audio frequency amplifier 45through the conductor |25 and through ground and the bypass condenser|23.

The anode |2| of the tube 6| preferably is connected to a positive pointon the voltage divider 93 through a resistor |21 which may have a valueof from one-fourth to one megohm. This resistor serves to maintain ahigh anode to cathode voltage when the tube 45 is blocked because of anyleakage current taken by tube 6|. The effect of the use of resistor |21in the circuit is to make the noise suppressor more rapid and positivein operation.

When there is no incoming signal, the plate impedance of the biasingtube 6| is maintained at a very high value by means of the control grid||5 which has a high negative potential sufficient to block the tube.'I'his high negative grid potential is obtained by connecting thecontrol grid ||5 to the lower end of the resistor R1, preferably througha resistor |29, this end of resistor R1 being identified as point B.Resistor |29 may be given a value of approximately one-half megohm.

The negative voltage applied to the control grid l5 from the point B isnot critical in value, approximately 20 volts on the grid, with noincoming signal, having `been found satisfactory when employing an RCA57 tube as the tube BI.

Point B is given the desired negative voltage with respect to ground,the above-mentioned 20 volts, for example, by the proper selection ofthe resistor values of the potentiometer In order to understand theoperation of the circuit more clearly, specific voltage values for thevoltage divider 93, the plate resistor 9|, and the potentiometer will begiven.

In one specific embodiment, the voltage divider 93 is connected toground at such a point intermediate its ends that the lower end of thevoltage divider is 150 volts negative with respect to ground, while theupper end of the voltage divider is 250 volts positive with respect toground. With no incoming signal, and a plate resistor 9| having a valueof from 50,000 to 100,000 ohms, the plate current of the pentodeproduces a voltage drop in the plate resistor 9| Ofsuicient magnitude tomake the upper end of the resistor 9| and potentiometer III (identifiedas point A) 25 to 50 volts positive with respect to ground. The point Bis then approximately 20 Volts negative with respect to ground, and theupper end of kresistor R5 (identified as point C) is still more negativeWithrespect to ground.

Thus, with no incoming signal, a constant biasing voltage is appliedfrom the point B to the control grid I I5 of the biasing tube 6|, abypass condenser I3I being connected between the control grid II5 andthe cathode ||3 for preventing audio frequency voltages or other ripplevoltages from being applied to the grid.

The biasing voltage applied to they control grid II5 when there is nosignal input is suflicient to block the tube 6|, thereby making itsplate resistance so high that the audio frequency amplifier tube 45 islikewise blocked. The blocking potential applied to tube 45 may becomeas high as 80 volts, this being much higher than necessary to block a.tube such as an RCA56 which has been found satisfactory for use with theRCA57.

When the receiver is tuned exactly to an incoming signal, anintermediate frequency voltage is impressed upon the input circuit ofthe pentode section 12, whereby the voltage drop in the plate resistor9| is decreased in value, causing the point B to assume a valuesufficiently less negative to unblockrthe biasing tube 6| and greatlylower its plate impedance. This causes a normal biasingvoltage to beapplied to the grid 53 and the audio frequency amplifier 45 isimmediately changed to its normal condition for an amplifier.

It should be noted that in case the point B becomes positive withrespect to ground, it will cause vgrid current to flow through theresistor |29 so that the grid I I5 will be maintained at substantiallyzero bias.

As fully explained in my above-mentioned patent, the characteristics ofthe pentode 6I are such that the plate impedance of the tube remainssubstantially constant within the' range of voltage applied tothecontrol grid during the time the amplifier 45 is effective. 'I'hisrange of voltages is determined by the adjustment of. the triggeramplifier 65. In a preferred adjustnient the potential of point B ischanged instantly from approximately 20 volts negative to about 3 Voltsnegative upon reception of a signal.

Further slight voltage changes may be caused by variations in thestrength of an incoming signal but they will not cause changes in theimpedance of tube 6|. For example, if the tube 6I is an RCA57, its plateimpedance will remain substantially constant for control grid voltagesranging from about 4 volts negative to zero and, since the .variationinplate voltage of tube is much greater than is necessary for thecontrol of grid II5, considerable variation in signal may take placewithout affecting the bias on tube 45.

The above-described circuit may be made so selective that it is animpractical circuit to employ if the receiver is to be tuned in theordinary way. To overcome this diiculty and also to increase the ease oftuning, even when the receiver is not unusually selective, I provide atuning control circuit whichincludes a control tube |33 and a magneticbrake |35.

The control tube |33 includes a cathode |31 which is connectedtonground, a control grid 39 which is connected to the upper end of. theresistor R5 through a grid leak resistor |4I (which may have aresistance of the order of one megohm), and an anodey |43 which isconnected through a conductor |45, the magnet coil |41 of the brake |35,and a conductor |49 to a positive point on the voltage divider 93. Thecontrol grid |39 is also connected to the point A 'through a condenserI5| which may have a value of approximately 0.1 microfarad. f

So long as no signal is being received, as when the receiver is tunedbetween stations, the point C applies aynegative potential to thecontrol grid |39 which is sufficient to block the tube |33 so that thereis no plate current flow through the brake winding |41, and the brake|35 is deenergized.

There is, however, a small iiow of current through the magnet winding|41, which flows in a direction opposite to the flow of plate current,for the purpose of removing residual magnetism from the brake after itis de-energized. This iiow of current may be traced from the positiveend of the voltage divider 93, through a resister |53 (which may have aresistance of. the order of 100,000 ohms) to the conductor |45, andthrough the magnet winding |41 and the conductor |49 to the positivepoint on the voltage divider 93.

The instant the receiver is tuned to the car.-

-rier wave of an incoming signal, the voltage applied to the controlgrid |39 of the brake control tube I 33v is changed. If it is changedsufciently, plate current flows through the magnet coil |41 of the brake|35 and the brake is instantly energized, thereby clamping the rotors ofthe tuning condenser II, I3, and I1, and the tuning knob |55, in theexact position at which the signal is properly tuned in.

In the apparatus illustrated in Fig. 1 the control grid of the brakecontrol tube |33 is so connected to the potentiometer II I that thebrake |35 is automatically released `a predetermined length of timeafter it has been energized. When employing this automatic releasecircuit the receiver is tuned by rotating the tuning knob |55 ratherslowly in case it is desired to tune in th next incoming signal.

As soon as the receiver is tuned in this manner to the carrier of thenext incoming signal, the potential of the point A becomes morepositive, and this more positive potential is transferred through thecondenser I5I tothe control grid |39 of the tube |33. 'I'hecomparatively slow rotation of the tuning knob |55 gives suflicient timefor the control grid |39 to assume a potential such that plate currentflows through the magnet coil |41 and the brake is energized. This locksthe tuning condensers II, I3, and I1 securely in position, and theoperators fingers may slip on ther tuning knob if further rotation isattempted. After an instant (one half to two seconds), the grid |39again assumes the potential of the point C which is sufficientlynegative to block the tube |33 even when there is an incoming signal.This releases the brake |35, and the receiver may be tuned to the nextstation in the same manner.

If it is desired to tune through one or more stations without having thebrake operate, the tuning knob is rotated more rapidly so that thepotential at A cannot increase appreciably because of the condenser |09and the control grid I 39 does not acquire the necessary more positivepotential for operating the brake each time the receiver is tuned to. acarrier wave.

It will be understood that, in general, the band pass filters 23 and 21will be designed to pass the intermediate frequency carrier and theupper and lower side bands, the pass range of each filter ,being made sonarrow for selectivity purposes that the intermediate frequency carriermust fall at the mid-point of the pass range in order to avoid thecutting off of a side band. It is essential, therefore, that thereceiver be tuned accurately to an incoming signal so that theintermediate frequency carrier falls at the proper point in the passrange of the filters, as otherwise the quality of the received signalwill be impaired. If the transformer 61 in the output circuit of tube 65is tuned sharply enough to the intermediate frequency, the magneticbrake |35 will be operated only when the receiver is tuned in accordancewith the above requirements for high quality reception.

In practice, it is found both difcult and eX- pensive to provide acontrol circuit for the magnetic brake which is so sharp that theselectivity of the circuit is itself sufficient to prevent the magneticbrake from being operated too soon if strong signals are impressed uponthe control circuit. This will readily be understood since the brake isoperated when the plate current of tube 65 reaches a predeterminedvalue. If signals of varying strength are impressed upon the inputcircuit of tube 65, a very strong signal at a frequency on one side ofthe resonant curve for the transformer 61 might produce the same platecurrent flow as a weaker signal at the intermediate frequency which islocated at the middle or peak of the resonant curve.

It has been found that the magnetic brake |35 may be made to operate atthe proper time when employing a control circuit having reasonably sharpselectivity if the strength of all signals applied to the controlcircuit is held substantially uniform. A convenient way of accomplishingthis is to provide the receiver with an automatic volume control circuitor AVC circuit which is applied to the portion of the receiver precedingthe point supplying voltage to the brake control circuit.

While various forms of automatic volume control may be utilized, thecircuit illustrated in the drawings is simple and effective inoperation. The AVC circuit includes one of the diode rectifiers |6| ofthe vacuum tube 29, this diode comprising the cathode 35 and a plate|63. The plate |63 is connected through a filter resistor |65 and lavolume control resistor |61 to ground. The junction point of theresistors |65 and |61 is connected to the control grids of the tubes 1and 2| through resistors |69, |1|, and |13, respectively. A ltercondenser |15 is connected between the grid end of the iilter resistor|65 and.l ground.

The cathode 35 is connected to the negative end of the voltage divider93 through a cathode resistor |11. 'I'his resistor is bypassed by meansof an audio frequency bypass condenser |19.

The operation of the AVC circuit is as follows: Assuming that a strongsignal is being received, there is then maximum flow of current throughthe second detector resistor 31 and the control grid 43 is at a ratherhigh negative potential, whereby the plate current of tube 29 is smalland there is a small voltage drop in the cathode resistor |11;Therefore, a large part of the voltage across the lower section I 8| ofthe voltage divider 93 is impressed upon the diode rectifier I6 Thiswill be seen by noting that the grounded point of the voltage divider 93is connected to the plate |63 of the diode rectier |6| through groundand through the volume control resistor |61, this grounded point beingpositive with respect to the cathode 35. The voltage across the voltagedivider section |8| is large compared with the voltage drop in thevolume control resistor |11 under the conditions assumed, so thatalthough the two voltages are in opposition, the greater part of thevoltage across the voltage divider section IBI is applied to therectifier |6|.

This results in a comparatively large flow of current through theresistor |61, with the result that the voltage drop in resistor |61maintains the control grids of the tubes 1, and 2| highly negative withrespect to their cathodes, whereby their gain is held at a low value.

If the strength of the received signal becomes less, the control grid 43becomes less negative, the plate current and the voltage drop in theresistor |11 increase, and a lower voltage is impressed across therectifier |6|. As a result, the current ow through the volume controlresistorv |61 decreases and the control grids of the tubes 1, and 2|become less negative, whereby the gain of the receiver is increased tobring up the signal strength.

From the above description it will be apparent that the output of thetransformer 21 remains substantially constant in amplitude so thatsignals of uniform amplitude are applied to the brake control circuitwhereby it will function uniformly regardless of the strength ofincoming signals. Thus, the tuning condensers will never be stopped bythe brake before they have been rotated to the proper position.

The above described automatic volume control circuit is described andclaimed in my application. Serial No. 640,946, filed November 3, 1932,and assigned to the same assignee as this application.

While various forms of magnetically operated brakes may be employed inmy automatic tuning circuit, the brake |35 shown in detail in Figs.,2and 3, has been found to be especially effective.

t comprises a U-shaped core |83 of magnetic material having a pair ofmagnet windings |41 on the legs of the core. The ends. of the core legsare flattened on one side to permit their use as a braking surface. f

A flat bar |81 of magnetic material is. mounted opposite the at surfacesof the core legs by means of hinge supporting members |89 which permitthe bar or armature |81 to rest against a disc segment |9|. 'I'hesupporting members |89 may be supported from a bar |90 of non-magneticmaterial secured tothe core |83. l The disc segment |9| is fastened tothe shaft of the tuning condensers with the edge thereof positionedbetween the flat surface of the magnet core |83 and the armature |81. Ina preferred embodiment, the disc segment |9| is mounted upon the tuningdial |93 at the outer edge of the dial. This disc segment |9| ispreferably made of magnetic material in order tov increase theeffectiveness of the brake.

An inspection of Fig. 3 willshow that the air gaps in the brakesmagnetic circuit arey very short so that the reluctance of the circuitis low, and the brake Very efficient. Referring to Fig. 4, there isshown a manually operable release for the magnetic brake |35, thisrelease being actuated by pushing in on a tuning knob |95. In Figs. 1and 4, like parts are indicated bythe same reference numerals. In thecircuit shown in Fig. 4, the control grid |39 of the tube |33 isconnected to a point D on the poten` tiometer which is sufficiently lessnegative than the point C to permit the control grid |39 becomingpositive, or nearly so, upon the reception of an incoming signal wherebytube |33 is unblocked and the brake is energized.

It will generally be desirable to have the tube |33l and the biasingtube 6| start drawing plate current at the same time. Therefore, thepoint on the potentiometer to which the control grid 39 should beconnected depends upon the relative characteristics of the two vacuumtubes. If the tube |33 Were the same type as the tube 6|, the controlgrid |39 preferably would be connected to the same point as the controlgrid of tube 6|, that is, to the point B.

Although the condenser |5| has been shown in connection' with the manualbrake release, it may be omitted if desired. Its use, however, willcause the magnetic brake to respond somewhat more rapidly to thereception of an incoming signal than it would otherwise since theincreasing potential of point A will be applied momentarily to the grid|39.

The tuning knob |95 of the receiver is slidably mounted upon a shaft |91in non-rotatable relation therewith, and provided with a flange |99 foractuating a switch when the tuning knob is pushed in. 'I'he tuning knobis normally held pushed out away from the panel of the receiver,indicated in dotted lines at 203, by means of a spring 205 as indicatedin the drawings.

The tuning knob shaft |91 may be connected directly to the tuning dialand tuning condenser shaft, or it may be connected thereto through afriction drive. In the drawings, the two shafts are shown mechanicallyconnected through a friction clutch 201, comprising two friction plateswhich are held against each other in any suitable manner. Intuning thereceiver, the tuning knob |95 is rotated While it is in the out positionindicated in Fig. 4. As soon as the receiver is tuned to an incomingsignal, the bias on the control grid |39 is so reduced in value thatplate current flows through the magnetic brake |35 and the tuningcondensers are locked in position. Any further rotation of the tuningknob |95 will cause the friction clutch 201 to slip and the tuning ofthe receiver will not be changed.

In order to tune the receiver to the next incoming signal, the tuningknob is pushed in to close the switch 20|. This places a high negativepotential on the control grid |39, thereby blocking the tube |33 andreleasing the brake |35. 'Ihe tuning knob |95 is then rotated far enoughto tune the receiver away from the station that was being received,which action will cause the point D to become sufficiently negative toblock the tube |33; 'Ihe tuning knob is then permitted to move back toits original position away from the panel 203, thereby opening theswitch 20| and bringing the receiver to its former condition. By furtherrotation of the tuning knob, the next incoming signal may be tuned in,thus causing the brake to be energized again.

If desired, a lamp may be substituted for the magnetic brake as a devicefor indicating when the receiver is tuned exactly to the desired radiostation. For example, a neon lamp 209 may be supported in the rear ofthe tuning dial |93, as shown in Fig. 4', and so connected that it maybe included in the' plate circuit of tube |33 by moving a switch arm 2||to its middle position.

It will be noted that when the switch arm 2| is in the middle position,the magnetic brake |35 is disconnected so that it cannot operate.Therefore, the release switch 20| has no function where the lamp aloneis used. In tuning the receiver, the receiver is tuned until the lamp209 lights up, thus showing that the receiver is accurately tuned to anincoming signal.

In one form of my invention, the lamp 209 may be used as described abovewith the noise suppressor omitted. In this form, the signal may be heardbefore the receiver is tuned Ato the point where the signal isundistorted but the operator Will know that the receiver is not properlytuned unless the lamp: has become lighted. The main feature of areceiver designed in this manner is that the circuit which controls thelamp 209 is sharply tuned to the intermediate frequency and, therefore,is much more sharply tuned than the signal channel.

If preferred, a signal lamp may be employed in combination With themagnetic brake. Thus a lamp 2|3 is positioned behind the tuning dial |93or in -any other suitable location and provided with any suitable sourceof current, such as a battery 2|5. A relay 2|'1 may be connected in theplate circuit of tube |33 by closing a switch 2|9. When the receiver istuned exactly to an incoming signal, the relay 2 1 is actuated and thelamp 2|3 is lighted.

In some cases, and particularly where the release mechanism of Fig. 4 isemployed, it may be preferred to have the radio receiver respond tostrong incoming signals only, such as those of local stations, wherebythe brake will be actuated by a comparatively small number of radiostations. This result can be obtained by providing any suitable form ofsensitivity control for the receiver. In the circuit shown in Fig. 1,such a control is provided by making the self-biasing resistors of tubesI, 1, and 2| variable. The resistance values of these resistors may bevaried simultaneously by means of a common control device indicated bythe dotted line 22|.

The use of the magnetic brake in combination with the noise suppressorhas a further advantage, in addition to those mentioned in the foregoingdescription. Because of an appreciable time lag in the action of thenoise suppressor, an operator might easilytune through a desired stationbefore the suppressor released if the brake were omitted. It will benoted that a considerable part of this time lag is introduced by theresistor |29 and condenser |3|, the circuit of condenser |3| h-aving afairly large time constant since condenser |3| must be large enough tobypass audio frequencies, preferably having a capacity of the order of0.25 microfarad.

'Ihe control circuit for the brake may be, and is in the circuit abovedescribed, given a time constant less than that of the noise suppressorcontrol circuit. Thus the brake responds to an incoming signal and stopsthe tuning condensers before the noise suppressor releases.

Various other modifications may be made in my invention withoutdeparting from the spirit and scope thereof and I desire, therefore,that Cil ually operable tuning knob, means 'for making a movement ofsaid tuning knob effective normally to produce a corresponding .movementof said tuning element, and means responsive to the reception of asignal for making said tuning knob ineective to produce a movement ofsaid tuning element.

2. A radio receiver for the reception of modulated carrier waves, saidreceiver comprising a demodulating device and converting means forconverting the carrier Wave of an incoming signal to a predeterminedfrequency, said means including a tunable oscillator, lter means fortransferring said converted carrier wave to said demo-dulating device,said lter means having a pass range sufciently wide to pass theconverted carrier having said predetermined frequency and at least oneside band thereof, a control circuit coupled to receive energy from theoutput of said converting means and sharply tuned to said convertedcarrier frequency, means responsive to the energy passed by said controlcircuit upon the reception of a si-gnal for controlling the tuning ofsaid oscillator, a manually operable tuning knob, means for making amovement of said tuning knob effective normally to produce acorresponding change in the tuning of said oscillator, and said meanswhich is responsive to the energy passed by the contr-ol circuit uponthe reception of a signal being operative to render the tuning knobmovement ineffective to produce a change in the tuning of theoscillator.

3. In -a radio receiver tunable over a predetermined frequency range, avariable tuning device, sensitivity control means for adjusting theresponse of said receiver to signals of predetermined strength, meansfor rendering a portion of sai-d receiver inoperative until the receiveris tuned to an incoming signal of said predetermined strength, and meansresponsive to the reception of said incoming signal for automaticallypreventing undesired movement of said tuning device, said second andlast-named means having differing time constants of operation wherebythe tuning movement is stopped in advance of the release of theinoperative condition of the receiver.

4. In a radio receiver, a signal selecting circuit, a variable tuningelement in said selecting circuit, a manually operable tuning knobcoupled to said tuning element, means including a magnetically operatedbrake for locking said tuning element in position in response to thereception of a signal, and means for releasing said brake by apredetermined movement of said tuning knob.

5. A magnetic brake for a radio tuning device, said brake comprising adisc segment of magnetic material coupled to said tuning device, aU-shape core positioned with its ends opposite one side of said discsegment, a magnet winding on said core, and an armature bar of magneticmaterial movably supported on the opposite side of said disc segment andopposite the ends of said core.

6. In a radio receiver tunable over a predetermined frequency range, avariable tuning device, a noise suppressor circuit comprising means forrendering a portion of said receiver inoperative when the receiver isnot tuned to an incoming signal and for rendering said portion operativein resp-onse to tuning said receiver to an incoming signal, and acontrol circuit comprising means responsive to said receiver being tunedto an incoming signal for preventing undesired movement of said tuningdevice, said control circuit having a time constant which is shorterthan the time constant of said noise suppressor circuit.

7. In a radio receiver having a continuously variable tuning device, amanually operable tuning knob, means for making a movement of saidtuning knob effective normally to produce a corresponding movement ofsaid tuning device, means responsive to the reception of a signal formaking said'tuning knob ineffective to pro-duce a movement of saidtuning device, and adjustable means for making said last means effectiveonly when the strength of said incoming signal is above a predeterminedamplitude.

8. In a radio receiver having a tuning device, a magnetic brake for saiddevice, said brake comprising a magnetic core having a winding thereon,means for producing a flow of current through said winding in onedirection in response to said receiver being tuned to an incomingsignal, and means for producing a ow of current through said winding inthe opposite direction in response to the receiver being tuned away fromsaid signal.

9. In a radio receiver, a signal selecting circuit, a variable tuningelement in said selecting circuit, a tuning knob for said tuningelement, a magnetic control device for said tuning element, said controldevice including a winding, an electric discharge tube having a controlgrid and having an output circuit which is coupled to said winding, aresistor section, a condenser and a grid resistor connected in seriesacross said section, said grid being conductively connected to thejunction point of said condenser and said grid resistor, means normallyfor maintaining a voltage across said resistor section such that saidgrid is made sumciently negative to substantially block said tube, andmeans for so changing the voltage across said section in response totuning said receiver to an incoming signal that said tube is unblockedfor a period determined by the time constant of the circuit includingsaid condenser and said grid resistor.

10. In a radio receiving system, the combination of a plurality oftuning condensers, a shaft directly connected with and movable tooperate said condensers simultaneously thereby to tune said receivercontinuously through a predetermined frequency range, a plate directlycarried by said shaft, a magnetic brake for locking said plate, shaftand condensers when said brake is energized, means for energizing saidbrake including a circuit sharply tuned to a predetermined frequencywhereby said tuning condensers are locked in position when said receiveris tuned exactly to a predetermined frequency, and means for causingsaid brake to operate in response to a signal of a predeterminedstrength only in response to movement of said tuning shaft at a ratebelow a predetermined value.

1l. A radio receiver for the reception of a signal modulated carrierWave, said receiver having a signal channel response of sufficient widthto pass said carrier wave and at least one and not more than both sidebands, a signal selecting circuit having a movable tuning elementtherein, and means including a sharply selective signal circuit forabruptly locking said tuning element in response to the reception of asignal, said last circuit being responsive to said carrier wave and moreselective than said signal channel, and means for adjusting saidresponse to signals of a predetermined strength.

12. A radio receiver comprising a signal selecting circuit tunable overa predetermined fre- 'que'nc'y range, an intermediate frequency ampliercoupled to said selecting circuit, automatic volume control means forsaid amplifier, a tuning control circuit coupled to said amplifier toreceive signals therefrom at substantially constant amplitude, saidcontrol circuit being more selective than said signal selecting circuitto the same signal, means including a brake responsive to the energypassed by said control circuit for controlling the tuning of said signalselecting circuit, and means for automatically deenergizing said brakeafter it has been energized for a predetermined length of time.

13. A radio receiver comprising a signal selecting circuit tunable overa predetermined frequency range, an intermediate frequency amplifiercoupled to said selecting circuit, automatic volume control means forsaid amplier, a tuning control circuit coupled to said amplifier toreceive signals therefrom at substantially constant amplitude, saidcontrol circuit being more selective than said signal selecting circuitto the same signal, means responsive to the energy passed by saidcontrol' circuit for controlling the tuning of said signal selectingcircuit, said last-named means including a tuning device for saidfirstnamed circuit having a movable tuning element, a magnetic brake forsaid element and a control tube through which said brake is energized,said control tube having acontrol grid connected with said tuningcontrol circuit, and means in said last-named connection for causingsaid brake to operate only when said tuning element is moved relativelyslowly.

14. In a radio receiving system, the combination of a plurality oftuning condensers, a shaft directly connected with and movable tooperate said condensers simultaneously thereby to tune said receivingsystem continuously through a predetermined frequency range, a platecarried by said shaft, a magnetic brake for locking said plate, shaftand condensers when said brake is energized, means for energizing saidbrake including a circuit sharply tunedto a predetermined frequency,Whereby said tuning condensers are locked in position when said receiveris tuned exactly with said predetermined frequency, said circuitincluding an amplifier tube, automatic volume control means for applyingsignals of substantially constant amplitude thereto, means for deriving.a signal-responsive control vpotential therefrom, a control tube forthe brake operative in responsev to said potential, Aand means providinga predetermined time constant for the' operationl of said control tube'and brake.

15. In a radio receiving system, the combination of a plurality oftuning condensers, a shaft` directly connected with and movable tooperate said condensers simultaneously thereby to tune said receivingsystem continuously through a predetermined frequency range, a platecarried by said shaft, a magnetic brake for locking said plate, shaftand condensers when said brake is energized,- and means for energizingsaid brake' including a circuit sharply` tuned to a predeterminedfrequency whereby said tuning condensers are locked in position whensaid receiver is tuned exactly with said predetermined frequency, saidcircuit including an amplifier tube, automatic volume control means forderiving a signal-responsive control potential therefrom, and means forcausing said brake to operate when said tuning shaft is moved relativelyslowly, said last-named means including a signal rectifier, a controltube for the brake connected with said rectifier to receive acontrolling potential therefrom, and a filter in said connection forregulating the time constant of the operation of the control tube inresponse to change in the controlling potential.

16. In a radio signal receiving system, the cornbination with a variabletuning device therefor having a movable element, of a tuning controlcircuit including a control tube and an electromagnetic brake, oneelement of said brake being rigidly connected with said movable tuningelement to move therewith, means for energizing said brake to lock saidmovable tuning element against tuning movement, including anelectromagnet Winding for the brakeA in circuit with said control tubeto receive anode current therefrom, a grid circuit for said control tubehaving a time delay filter means therein, means for applying asignal-responsive control potential to said grid circuit through saidfilter, and automatic volume control means for controlling the amplitudeof said potential, whereby said last-named means may be more broadlyresponsive to a received signal of relatively high amplitude.

LOY E. BARTON.

